This invention relates to a method of providing an estimate of GPS time from non-GPS timing information present in communications signals transmitted from a base station to a mobile communications device, and to apparatus for the same.
It is well known to provide a GPS receiver in which replica GPS satellite pseudorandom noise (PRN) code signals are continuous generated and correlated with received GPS signals in order to acquire them. Typically, as the replica codes are likely to have a different code phase to those of the received GPS signals and also a different frequency due to Doppler shift between the receiver and orbiting satellites, a two dimensional code frequency/phase sweep is employed whereby such a sweep will eventually result in the incoming PRN code having the same frequency and code phase as that of the locally generated replica. If detected, the code is acquired and tracked, and the pseudorange information may be retrieved from which the position of the receiver may be calculated using conventional navigation algorithms.
It is further known to provide a mobile cellular telephone incorporating such a GPS receiver for the purpose of enabling operators of cellular telephone networks to determine the location from which a call is made and, in particular, for an emergency call to the emergency services. Of course for an emergency call, it is desirable for the call location to be available as soon as possible, however, from a xe2x80x9ccold startxe2x80x9d where the GPS receiver does not have access to up to date ephemeris data or even worse from a xe2x80x9cfactory cold startxe2x80x9d where the GPS receiver does not have an up to date almanac, the time to first fix (TTFF) can be anywhere between 30 seconds and 15 minutes.
In order to reduce the TTFF, a GPS receiver may be provided with base station assistance in order to acquire GPS signals more quickly. Such assistance may include the provision by the base station to the receiver of a precision carrier frequency reference signal for calibrating the local oscillator used in the GPS receiver; the data message for up to date satellite almanac and ephemeris data from which Doppler shift for satellites in view can be determined. With such assistance, it is possible to sweep only a narrowed range of frequencies and code phases in which the target PRN code is known to occupy, thereby reducing the number of code instances that need to be checked and thus reducing the time for code acquisition. Base station assistance is further described in U.S. Pat. Nos. 5,841,396 and 5,874,914 which are incorporated herein by reference.
In order to acquire GPS signals even more quickly, it is desirable for the GPS receiver to be in possession of an accurate estimate of GPS time before despreading any of the GPS signals. With an accurate estimate of GPS time, a rough estimate of the position of the GPS receiver location and knowledge of the GPS satellites orbits, it is possible to predict further aspects of the incoming GPS signals such as data bit polarities, the timing of data bit edges and code phases, so making the GPS signals easier to acquire.
U.S. Pat. No. 5,945,944 discloses an arrangement in which a mobile cellular telephone comprises a GPS receiver and derives GPS timing information from network timing information contained in transmissions received by the telephone from corresponding network base stations. This is especially convenient for networks such as those compliant to the IS-95 and CDMA2000 standards as their base stations include GPS receivers to ensure their synchronization. However, as acknowledged in U.S. Pat. No. 5,945,944, not all networks have such synchronized between base stations.
Lines 38 to 53 of column 16 of U.S. Pat. No. 5,945,944 purport to disclose a method of obtaining GPS time in an unsynchronized network. Specifically, it states that the problem of requiring that each GPS basestation has access to the timing information of the cell site is partially solved by having a series of cellular telephones located in each cell so that the absolute time for that cell may be coordinated between a GPS basestation and a remote unit, independently of the location of the remote unit within the cell. From this somewhat limited disclosure, it is not obvious how obtaining GPS time in such an unsynchronized network is in fact achieved.
It is therefore an object of the present invention to provided a method of providing an estimate of GPS time from non-GPS timing information present in communications signals transmitted from a base station to a mobile communications device, especially where the base station is one of several such basestations in a cellular telephone network which are not synchronized using GPS receivers.
In accordance with the present invention, such as a method is provided comprising the steps of: (i) in a second mobile communications device, receiving corresponding non-GPS timing signals present in communications signals transmitted by the base station (ii) in a GPS receiver of the second mobile communications device, obtaining at least one pseudorange to a GPS satellite and obtaining an estimate of GPS time; (iii) deriving a relationship between the receipt of timing signals received in step (i) and the estimate of GPS time obtained in step (ii); and (iv) from the non-GPS timing information received by the first mobile communications device and the relationship derived in step (iii), calculating an estimate of GPS time.
The above method enables an estimate of GPS time to be provided in the first mobile communications which maybe be used, for example, to rapidly acquire GPS signals in a GPS receiver of the first mobile communications device, or to provide a GPS time based time-stamp. In the case of the latter, the first mobile communications does not strictly require a GPS receiver.
In a preferred method, both the first and second mobile communications devices are synchronized with the base station in a manner which takes in to account variation in the transmission times of transmissions sent between the mobile communication devices and the base station. This would enable the estimate of GPS time to be provided independently of the location of both the first and second mobile communications devices.
Also, whilst a single pseudorange to a GPS satellite in combination with an estimate of the locations of the corresponding GPS satellite and the second mobile communications device can be used to obtain an estimate of GPS time, it is preferable that the GPS receiver of the second mobile communications device obtains at least four pseudoranges to respective GPS satellites as such location information would not then be required.
One would normally expected the calculation of step (iv) to be done in the first mobile communications device so as to enable the incoming GPS signals to be despread at the same time. However, it will be appreciated that a sample of the GPS signal (occasionally referred to as a snapshot) could be uploaded via the base station to a remote computer and, say if the snapshot time is time stamped with the non-GPS timing information, the calculation of (iv) and subsequent despreading of the GPS signal done at the remote computer.
The relationship between the receipt of non-GPS timing signals and the estimate of GPS time derived in step (iii) may be uploaded to a database held on a server located remote from the mobile communications devices, e.g. one held on a web-server and capable of being accessed by the first mobile communications device over the internet or, alternatively, one connected to a network system controller.
Ideally, the database is capable of being accessed not only by the first mobile communications device but by other mobile communications devices registered with the same base station. Also, where the base station is one of several such basestations of a cellular telephone network which are not synchronized using GPS receivers, the database preferably contains relationships of the type derived in step (iii) for at least two such basestations.
Further provided in accordance with the present invention is a corresponding system comprising a base station and first and second mobile communications devices as claimed in claims 14 to 25; and mobile communications devices as claimed in claims 26 to 29.